The Turn Of The Century Electrotherapy Museum http://www.electrotherapymuseum.com Micrometer Spark Gap Power Supply Pancake Coil powered by cigar-box tank circuit Box contains 1650V 10mA transformer and .0315 mfd condenser @ 7kV.   [ (7) .22 mfd  1kV condensers in series] Note the position of the conical adjustment - Gap is open 1/16". As the cone is pushed or pulled the gap is adjusted - a 3" portion of the cone adjusts the gap by a maximum of 1/16". Hints of what's inside... Internal view...